Conventionally, a power tool employs a lithium battery which can supply a large current. However, when the lithium battery is continued to be used in an over-discharge state, the life time of the battery becomes short and there arises a risk that the battery catches fire. Thus, the power tool of the related art is provided with an over-discharge detection means for stopping the supply of electric power to a motor from the lithium battery so that the lithium battery is placed in the over-discharge state (see Japanese Patent No. 3222951, for example).